1. Field of the Invention
The present invention relates to electrostatographic imaging and recording apparatus, and to assemblies in these apparatus for fixing toner to the substrates. The present invention relates particularly to a fuser member, and to a fusing surface layer for fuser members, in the toner fixing assemblies.
2. Description of Background and Other Information
Generally in electrostatographic reproduction, the original to be copied is rendered in the form of a latent electrostatic image on a photosensitive member. This latent image is made visible by the application of electrically charged toner.
The toner thusly forming the image is transferred to a substrate—also referred to in the art as a receiver—such as paper or transparent film, and fixed or fused to the substrate. Where heat softenable toners—for example, comprising thermoplastic polymeric binders—are employed, the usual method of fixing the toner to the substrate involves applying heat to the toner, once it is on the substrate surface, to soften it, and then allowing or causing the toner to cool. This application of heat in the fusing process is preferably at a temperature of about 90° C.–220° C.; pressure may be employed in conjunction with the heat.
A system or assembly for providing the requisite heat and pressure customarily includes a fuser member and a support member. The heat energy employed in the fusing process generally is transmitted to toner on the substrate by the fuser member. Specifically, the fuser member is heated; to transfer heat energy to toner situated on a surface of the substrate, the fuser member contacts this toner, and correspondingly also can contact this surface of the substrate itself. The support member contacts an opposing surface of the substrate.
Accordingly, the substrate can be situated or positioned between the fuser and support members, so that these members can act together on the substrate to provide the requisite pressure in the fusing process. In cooperating, preferably the fuser and support members define a nip, or contact arc, in which the substrate is positioned or resides, and/or through which the substrate passes. Also as a matter of preference, the fuser and support members are in the form of fuser and pressure rollers, respectively. Yet additionally as a matter of preference, one or both of the fuser and support members have a soft layer that increases the nip, to effect better transfer of heat to fuse the toner.
In contacting toner on the substrate, the surface of the fuser member imparts a surface texture to the toner, and accordingly to the image formed thereby. This surface texture determines the degree of image gloss; differences in the texture of the toner results in varying gloss levels.
With glossy images formed by the indicated fusing process, there are certain disadvantages. Subtle defects can be seen in these images, particularly defects that cause subtle variations in gloss.
However, matte images, or low gloss images, minimize objectionable glare, and they reduce or even hide various image defects, such as oil defects. Particularly, low gloss images can make subtle defects invisible.
Where the receiver is paper, low gloss images are further advantageous in that they do not produce objectionable differential gloss due to nonuniformity in the paper height. Varying types of paper have different levels of roughness, and glossy images produced on rough paper often produce differential gloss. This is particularly a problem in high speed digital printing, where high paper speeds require short fusing times. In the faster printers, when the image is high gloss there is often not time to allow a uniform gloss image on rough papers. As the final operation for fixing the image to the receiver, fusing usually produces the final image surface. Thus, altering the fusing process can be necessary to obtain a desirable image property, such as low image gloss.
Heavily filled silicone rubber, used for fuser member surfaces, is known to produce high quality fused images with the desired low gloss. The polysiloxane elastomers have relatively low surface energies and also relatively low mechanical strengths, but are adequately flexible and elastic. Unfortunately, silicone rubbers wear easily when employed for this purpose; after a period of use, the action of the paper or other media passing through a high pressure nip wears a polysiloxane elastomer fuser surface. The silicone rubbers' low wear resistance as fuser member surfaces accordingly limits fuser member life. Further, although treatment with a polysiloxane release fluid during use of the fuser member enhances its ability to release toner, the fluid causes the silicone rubber to swell. This fluid absorption is a particular factor that shortens fuser member life; fluid treated portions tend to swell and wear and degrade faster. Fuser members with polysiloxane elastomer fusing surfaces accordingly have a limited life.
Fluorocarbon materials also have low surface energies, and, like silicone rubbers, are used as release surface materials for fuser members. Polyfluorocarbons employed for this purpose include nonelastomeric fluorocarbon materials, or fluoroplastics, and fluoroelastomer materials. However, there are disadvantages associated with the use of both.
U.S. Pat. Nos. 5,363,180, 5,508,138, 5,519,479, 5,649,273, these four patents being incorporated herein in their entireties, by reference thereto, disclose a fixing device which is for use in an electrophotographic apparatus, and which includes a fixing roller. The fixing roller can have a surface layer, with particles harder than the layer dispersed therein. This surface layer can comprise a heat resistant fluororesin such as ethylene tetrafluoride, or a fluorine type heat contracting resin such as tetrafluoroethylene-perfluoroalkylvinylether copolymer.
In fact, the fluorocarbon resins like polytetrafluoroethylene (PTFE), and copolymers of tetrafluoroethylene (TFE) and perfluoroalkylvinylether (PFA), and fluorinated ethylene propylene copolymers, have excellent release characteristics due to very low surface energies. They also are characterized by high temperature resistance, excellent chemical resistance, and low wear (high abrasion resistance).
However, fluorocarbon resins are less flexible and elastic than polysiloxane elastomers, and are unsuitable for producing high image quality images. Fluorocarbon resins typically have a high modulus, and cannot evenly contact rough papers; they therefore provide varying gloss within the same image. The high modulus also tends to produce images with objectionable mottle, and contributes to high gloss; specifically, with both a smooth surface and high modulus, there will be high gloss in addition to the objectionable mottle.
Fluoroelastomers also have low surface energy. They have excellent wear resistance as fusing member surfaces, providing better durability in this regard than the polysiloxane elastomers, and unlike the silicone rubbers, do not swell when in contact with polysiloxane release fluids. However, due to their relatively greater hardness as compared with that of the silicone rubbers, fluoroelastomers also typically produce objectionably high gloss images.
It would accordingly be desirable to provide a fusing member fluoroelastomer surface which retains the indicated advantages of fluoroelastomers, while also producing low gloss and high quality images as are achieved with silicone rubbers. It would further be desirable that obtaining the requisite low gloss not depend on the properties of the substrate, but rather be accomplished over a range of substrate surface roughness.
Moreover, it is known that the presence of heat conducting inorganic filler particles, in the fluoroelastomer fusing surface layers of fuser members, provides high energy sites for removing toner from the substrate. Increasing the amount of heat conducting filler content in the fusing surface layer, by providing more reactive sites for the toner, therefore increases toner offset, and accordingly also increases contamination of the fuser member. Thus it would yet additionally be desirable that, where heat conducting inorganic filler particles are present in the fluoroelastomer fusing surface layer, the amount of the filler be minimized.